Powdery Mildew

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Importance

Powdery mildew is a very recognizable and common fungal disease found on several plants in Minnesota. On trees and shrubs, powdery mildew rarely causes serious damage to its host. The disease can significantly reduce the ornamental value of plants grown for their appearance like roses and purple leafed ninebark shrubs.

Pathogen and susceptible plants

There are many species of powdery mildew fungi and they affect thousands of plant species. The fungi that cause powdery mildew are generally host-specific and only infect plants from the same genus or family. Therefore the fungus on your ash tree will not infect your viburnum bush. Only a few species of powdery mildew fungi are capable of causing disease in multiple trees and shrubs. In many cases, however, one species of tree or shrub is susceptible to several different species of powdery mildew fungi.

Table 1. Trees and shrubs on which powdery mildew has been identified in Minnesota

Apple
Ash
Azalea
Birch
Buckeye
Barberry
Cotoneaster
Crabapple

Catalpa
Dogwood
Elm
Euonymus
Hawthorn
Ironwood
Lilac
Linden

Maple
Magnolia
Oak
Serviceberry
Spirea
Rhododendron
Rose
Viburnum

M. Grabowski, UMN Extension

Powdery mildew infection on the lower leaves of a young oak

Identification

M. Grabowski, UMN Extension

This single ninebark shoot is completely infected with powdery mildew, and likely grew out of a bud infected by the fungus.

Powdery mildew appears as superficial growth on plant surfaces and is seen as white to gray powdery spots, blotches or felt-like mats on leaves, stems and buds.

Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.

Disease is often most severe on young leaves, water sprouts and green shoots.

Once severely infected, leaves may turn yellow and fall prematurely during the growing season.

In some plants, leaves turn purple to red around the infection.

In late summer/early fall, tiny round orange to black balls form within white fungal mats.

Most prevalent when outdoor conditions consist of cool temperatures with high humidity; however, it can be seen in warm, dry conditions as well.

Disease is most severe on plants or plant parts in shaded areas with poor air movement (interior or lower branches).

Biology

Powdery mildew fungi survive winter as chasmothecium (previously known as cleistothecium), dark colored round fungal structures that contain spores and protect them through the winter. In spring, the chasmothecium rupture releasing spores that are spread by the wind. Some species of powdery mildew fungi survive the winter as a dormant fungus – or mycelium – in infected buds or shoot tips. In spring, the mycelium or spores start new infections on succulent, new growth. The fungus grows and becomes evident as the white powdery mat on the leaf surface. Throughout the growing season, additional spores are produced and spread by wind. These spores start new leaf spots on the plant and on neighboring plants.

Powdery mildew thrives in humid conditions but does not do well if leaves are wet from frequent rain or irrigation. Favorable conditions for powdery mildew commonly occur when cool nighttime temperatures are followed by warm day temperatures. Consequently, in Minnesota increased levels of powdery mildew occur in the spring and fall of the year.

Management

Powdery mildew does not significantly affect the health of the tree or shrub and does not require management; although, photosynthesis can be reduced in heavily colonized leaves.

Powdery mildew resistant cultivars are available for many ornamental shrubs. Choose disease resistant cultivars for new plantings or as replacement plants.

Do not overcrowd plants – use size at maturity as a spacing guide when planting.

Prune the tree or shrub to increase light penetration and improve air circulation throughout the canopy.

Many plant species are most susceptible when new, succulent shoots and leaves are being produced; for such species, pruning and fertilizing stimulate growth and trigger or prolong powdery mildew.

- Do not fertilize trees and shrubs suffering from powdery mildew infections, unless it is recommended by a soil test to correct a nutrient deficiency.

- Remove only severely infected and damaged shoots in the summer to reduce spread and overwintering of the fungi within the canopy. Avoid excessive pruning of infected plants.

Fungicides

Fungicides can be used to protect highly susceptible and prized ornamental shrubs like roses or ninebarks. Fungicides prevent infection and some fungicides eradicate small infections that have started to form. For shrubs that have a history of disease, fungicides can be applied before disease symptoms appear. Alternatively, scout plants regularly. When a few powdery mildew leaf spots appear, apply fungicides to protect the remaining leaves. In both cases, fungicide sprays will need to be repeated according to label instructions to protect plants throughout the growing season.

Chemical treatments include:

Thiophanate methyl

Chlorothalonil

Sulfur (not for sulfur sensitive plants like viburnum)

Potassium bicarbonate

Always completely read and follow all instructions on the fungicide label.